Infants with an older brother or sister with autism are at increased biological risk for an autism spectrum disorder. To date, there has been no reliable way to predict which of these younger siblings will later be diagnosed with ASD. This large, prospective study will attempt to identify variations in vocal, motor, communicative, emotional, and related areas of development in infants that can serve as predictors of a future diagnosis of autism. The many facets of temperament and emotional expression of these high risk children will be compared to infants with no known risk in search of differences that discriminate between infants who will receive a positive screen for an autism spectrum disorder at 24 months and those who will not. Extensive coding and analysis of videotapes will be used to assess behaviors such as social avoidance, social referencing, strategies for emotional self-regulation, startle responses, use of gestures and vocalizations, and manifestations of distress. What this means for people with autism: The knowledge that younger siblings of autistic children are at increased risk can be incredibly stressful for parents. The presence of risk factors could help parents take action sooner, and their absence could put parents' minds at ease. Moreover, the study could lead to earlier ASD screening for all infants and allow for even earlier treatment than is currently possible.